Osteoarthritis (OA) is thought to be caused by a combination of intrinsic vulnerabilities of the joint, such as anatomic shape and alignment, and environmental factors, such as body weight, injury, and overuse. In the hip, for example, it has been postulated that much of osteoarthritis is due to developmental anatomic deformities. Recent studies have demonstrated that more subtle anatomic abnormalities, such as acetabular retroversion, acetabular over-coverage, and decreased head-neck offset of the femoral head-neck junction are also important anatomic variants of the hip joint that may lead to pain and OA.
Femoral Acetabular Impingement (FAI) is often classified into two distinct entities, namely, cam impingement and pincer impingement. Cam impingement results from pathologic contact between an abnormally shaped femoral head and neck with a morphologically normal acetabulum. This pattern of impingement is characterized by a femoral head-neck junction which is not spherical anteriorly and has increased radius of curvature. As the hip flexes, this abnormal region engages the anterior acetabulum. The resultant shear forces which result from this contact produces the characteristic anterosuperior chondral injury and associated labral tear. The second type of FAI, pincer impingement, is the result of contact between an abnormal acetabular rim and a typically normal femoral head-neck junction. This pathologic contact is the result of abnormal anterior acetabular “over coverage.” This results in decreased joint clearance and repetitive contact between the femoral neck and acetabulum. Ultimately, this repetitive contact causes degeneration of the anterosuperior labrum much like in cam impingement. The injured labrum subsequently may become calcified, further worsening the anterior “over coverage.” Additionally, because the anatomic constraint in the native hip is so great, the contact can cause leverage of the head out of the acetabulum posteriorly contributing to a “contre-coup” injury to the posteroinferior acetabulum.
Cam and pincer impingement differ in mechanism, epidemiology, pathoanatomy and surgical management. However, it is not uncommon to see both of these lesions coexisting in a patient with FAI.
The goal of surgical intervention is to relieve the impingement by increasing hip clearance in flexion or some other motions as well as addressing the associated labral and chondral pathology. Surgery is tailored to the underlying anatomic abnormality. Cam type impingement, with prominence of the femoral head-neck region, is addressed on the femoral side with femoral neck osteoplasty or osteochondroplasty. The goal of femoral neck osteoplasty is to recreate the anatomic sphericity of the femoral head and to reduce the prominence of the femoral neck which abuts the anterior labrum and acetabulum. Conversely, pincer impingement lesions often require resection osteoplasty of the acetabular rim with repair of the labrum to its proper anatomic position. When these lesions coexist, osteoplasty of both the femoral head-neck junction and the acetabular rim is required.
Classically, the surgical approach to these lesions has been a formal open surgical dislocation including trochanteric osteotomy. This approach has been espoused for its ability to give an unobstructed 360° view of the femoral head and acetabulum.
Minimally invasive surgery (MIS) and arthroscopic techniques for osteoplasty cause much less morbidity and pain for the patient, and help to promote a much quicker recovery. However, they are significantly more difficult and even impossible for some surgeons to perform, mainly due to the reduced visualization and access in comparison to open techniques. Identifying the impingement zones is problematic due to, for example, the flexion of the hip joint and the interference of surrounding soft tissues. Moreover, it is extremely difficult to visualize the correct amount of bone that should be removed, and to easily verify this without removing too much bone.
In MIS, fluoroscopy is often needed and used to enhance visualization. However, introducing a fluoroscopic arm into the operating room places a great burden on the surgeon, operating room staff, and patient due to the logistic, ergonomic, and radiation issues.
Some have advocated simulating the patients' range of motion on pre-operative images such as computed-tomography (CT) scans or MRI, in order to the physician in planning surgery. However, this approach has several disadvantages. Pre-operative scans such as CT scans require costly imaging equipment and technicians, and they are time consuming for the surgeon. These simulation tools require the transfer and processing of images, using segmentations algorithms which are often not robust and difficult to use. In addition, the simulations cannot take into account the actual kinematics of the patient, and the effects of soft tissues on the patient's real range of motion. The surgeon is also burdened with having to register this preoperative plan to the patient in surgery.
Therefore, an intraoperative tool to help surgeons plan and perform oseteoplasty surgeries, such as, FAI in a more precise and less invasive manner would be an invaluable tool for surgeons and for patients.